Metastability still exist today.
Metastability period of opportunity do diminish because of the
technology is better, but it also augment because the operating
frequency augment.
It's not that prevalent in system where one clock domain is used because
to prevent it you just have to design by respecting the propagation time
between 2 flip-flop and respect the setup and hold time of each flip-flop.
If you have more 2 unrelated clock you are sure to have metastability
problem. Because since the 2 clock doesn't have the same
frequency/(phase if different origin) there is drift between the 2 and
you can expect metastability. Even if you calculate a 0.00001 % chance
of metastability per data line(between 2 Flip-flop each in is own clock
domain) you can expect, for example, a error rate of 10 error*per data
line/second at a frequency of 100MHz. That kind of error rate is
unacceptable in any IC.
To diminish the error rate for a data bus, the asynchronous fifo is the
solution commonly proposed. The interface is composed of 2 synchronous
part(data in-out and counter), a asynchronous dual port ram and
asynchronous fault tolerant control logic. That type of construction
diminish the error probability by and important factor since the
interface is asynchronous and is expected to behave rationally between 2
clock domain. The control stop the write operation and read operation to
happen in the same memory case at the same time, but allow it at a
different time or in a different memory space.
Nicolas Boulay wrote:
Does metastability still exist today ?
Metastability have a exponentialy decreasing probability for a given
length. The idea is that the time constant in the exponent for the
last process (<0.25µm) are very small (ps ?) and don't affect the
design anymore.
2007/4/10, Timothy Normand Miller <[EMAIL PROTECTED]>:
I've posted to SVN a new fifo design. It's kinda wasteful, but it's
designed for very high clock rates. It's an async fifo (meaning that
the two ends are on different clocks), and the cross-domain
communitation is one-hot (rather than gray-coded).
When I posted the original one, someone was kind enough to perform an
analysis to determine if RAM contents would also suffer from
metastability. I think this one will have the same problem, so I was
hoping that same kind person would please have a look at this one.
https://svn.suug.ch/repos/opengraphics/main/trunk/rtl/fifos/onehot_fifo_32.v
Thanks!
(Oh, it doesn't quite work right yet. I'm in the process of
debugging it.)
--
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Open Graphics Project
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